System and method for narrow bandwidth digital signal processing

ABSTRACT

The present invention provides methods and systems for narrow bandwidth digital processing of an input audio signal. Particularly, the present invention includes a high pass filter configured to filter the input audio signal. A first compressor then modulates the filtered signal in order to create a partially processed signal. In some embodiments, a clipping module further limits the gain of the partially processed signal. A splitter is configured to split the partially processed signal into a first signal and a second signal. A low pass filter is configured to filter the first signal. A pass through module is configured to adjust the gain of the second signal. A mixer then combines the filtered first signal and the gain-adjusted second signal in order to output a combined signal. In some embodiments, a tone control module further processes the combined signal, and a second compressor further modulates the processed signal.

CLAIM OF PRIORITY

The present application is a continuation of U.S. patent applicationSer. No. 15/044,768 filed on Feb. 16, 2016, which is set to mature intoU.S. Pat. No. 9,741,355, which is a continuation of U.S. Pat. No.9,264,004, filed on Sep. 20, 2013, which is based on and claims priorityunder 35 U.S.C. Section 119(e) to Provisional patent application havingSer. No. 61/834,071 and a filing date of Jun. 12, 2013, all of which areexplicitly incorporated herein by reference, in their entireties.

FIELD OF THE INVENTION

The present invention provides for methods and systems for digitallyprocessing narrow bandwidth digital signals. Specifically, someembodiments relate to digitally processing an audio signal where theaudio content is restricted to only a few octaves.

BACKGROUND OF THE INVENTION

Narrow bandwidth audio processing has a variety of applications,including voice communications in extreme first responder environments,auscultation of various body sounds, hydrophones, telecommunications, orunusual speaker/microphone situations where the available information isof very poor quality. In these situations, the audio content isgenerally restricted to only a few octaves.

Although traditional, full spectrum audio enhancement systems andmethods are capable of enhancing the sound quality of narrow bandwidthaudio content, these standard configurations tend to be underutilizedand fall short of achieving stellar quality enhancements. Accordingly,it would be beneficial to improve audio processing and enhancement fornarrow bandwidth audio content throughout various applications, in orderto produce a clear and easily interpretable audio output. Further, itwould be beneficial to allow a user to calibrate the audio processingfor different applications having different narrow bandwidth frequencyranges.

SUMMARY OF THE INVENTION

The present invention meets the existing needs described above byproviding for a system and method for narrow bandwidth processing of anaudio signal, in order to create a clear and easily interpretable audiooutput.

Accordingly, in initially broad terms, an audio input signal is firstfiltered through a high pass filter. The high pass filter may comprise aresonant high pass filter with a first frequency. As such, thefrequencies of the audio input signal above the first frequency arepassed through to a first compressor as a filtered signal. The firstcompressor comprises dynamic gain compression capabilities in order tomodulate the filtered signal.

In some embodiments, the modulated signal is then transmitted to aclipping module in order to create a partially processed signal. Theclipping module limits the gain of the modulated signal in order tocreate a partially processed signal. In other embodiments, the filteredsignal from the first compressor is transmitted as the partiallyprocessed signal, without the use of a clipping module.

In some embodiments, the partially processed signal is sent to atransmitter for transmission to a receiver over a communicationsnetwork, the receiver then transmits the partially processed signal to asplitter. In other embodiments, the partially processed signal is sentdirectly to the splitter without the use of a transmitter or receiver.

The partially processed signal is then split in the splitter into afirst signal and a second signal. The first signal is then filteredthrough a low pass filter. The low pass filter may comprise a resonantlow pass filter with a second frequency. The frequencies below thesecond frequency are passed through. In a preferred embodiment, andcounterintuitive to expected function, the second frequency is selectedto be lower than the first frequency. This allows the resultant signalto be used to rebuild usable dynamics from the low frequencies. Thesecond signal is transmitted to a pass through module. The pass throughmodule may be configured to adjust the gain of the second signal. Thepass through module may simply pass through the second signal withoutany modification. Further, the gain adjustment may be a staticadjustment. The filtered first signal and gain-adjusted are thencombined in a mixer in order to create a combined signal.

In some embodiments, the combined signal is then processed by a tonecontrol module. The tone control module may comprise a parametricequalizer or other equalizer configured to fine tune the signal. In atleast one embodiment, the processed signal from the tone control modulemay then be modulated by a second compressor. The modulated signal isthen sent as the output signal. In other embodiments, the combinedsignal from the mixer may be directly sent as the output signal, withoutthe use of the tone control module or the second compressor.

Applications of the present invention may include voice communication,such as those in extreme first responder environments or where theavailable audio content is of poor or very poor quality. In theseapplications, the present invention may filter out background noise, andselectively enhance the frequency range of voice applications in orderto yield a clear and interpretable audio output. In these embodiments,the first frequency may be selected from the range of 2.5 kHz to 3.2kHz, and the second frequency may be selected from the range of 700 Hzto 1000 Hz.

Other applications may include hydrophones or sonar, such as those usedfor underwater or underground resource exploration, and appropriatefrequency ranges for such applications. In body auscultations, thepresent invention may also be used to enhance respiratory sounds, forinstance as part of an electronic stethoscope. The present invention mayalso be used to enhance heart sounds, for instance as part of a devicefor phonocardiogram or echocardiograph, as well as other detectionmethods. The present invention may also be embedded as part of, orconnected to a computer in order to analyze pre-recorded narrowbandwidth audio content. Accordingly, the first and second frequencyranges, as well as gain control and compression elements, may be set orcalibrated accordingly depending on the type of audio content.

These and other objects, features and advantages of the presentinvention will become clearer when the drawings as well as the detaileddescription are taken into consideration.

BRIEF DESCRIPTION OF THE DRAWINGS

For a fuller understanding of the nature of the present invention,reference should be had to the following detailed description taken inconnection with the accompanying drawings in which:

FIG. 1 illustrates a schematic of one embodiment of the presentinvention directed to a system for narrow bandwidth processing.

FIG. 2 illustrates a schematic of another embodiment of the presentinvention directed to a broadcast variation of the system of FIG. 1.

FIG. 3 illustrates a schematic of another embodiment of the presentinvention directed to a system for narrow bandwidth processing.

FIG. 4 illustrates a schematic of another embodiment of the presentinvention directed to a broadcast variation of the system of FIG. 3

FIG. 5 illustrates a block diagram of another embodiment of the presentinvention directed to a method for narrow bandwidth processing.

FIG. 6 illustrates a block diagram of another embodiment directed to abroadcast variation for the method of FIG. 5.

FIG. 7 illustrates a block diagram of another embodiment of the presentinvention directed to a method for narrow bandwidth processing.

FIG. 8 illustrates a block diagram of another embodiment directed to abroadcast variation for the method of FIG. 7.

Like reference numerals refer to like parts throughout the several viewsof the drawings.

DETAILED DESCRIPTION OF THE EMBODIMENT

As illustrated by the accompanying drawings, the present invention isdirected to systems and methods for narrow bandwidth digital signalprocessing.

As schematically represented, FIG. 1 illustrates at least one preferredembodiment of a system 100 for narrow bandwidth digital signalprocessing. Accordingly, and in these embodiments, the system 100generally comprises an input device 101, a high pass filter 111, a firstprocessing module 105, a splitter 121, a resonant low pass filter 122, apass through module 123, a second processing module 106, and finally anoutput device 102.

The input device 101 is at least partially structured or configured totransmit an input audio signal 201 into the high pass filter 111. Theinput audio signal 201 may comprise the full audible range, but will incertain applications comprise less than half of the 20 Hz to 20 kHzaudible range. The input device 101 may comprise at least portions of anaudio device capable of audio playback. The input device 101 forinstance, may comprise a stereo system, a portable music player, amobile device, a computer, a sound or audio card, or any other device orcombination of electronic circuits suitable for audio playback.

The high pass filter 111 is configured to pass through high frequenciesof the input audio signal 201, while attenuating lower frequencies,based on a first frequency. In other words, the frequencies above thefirst frequency are transmitted as a filtered signal 211 to the firstcompressor 112. In at least one embodiment, such as those for voiceapplications, the first frequency may be selected from a range between2.5 kHz and 3.2 kHz. The first frequency however, may vary depending onthe source signal. The first frequency may further be tunable by a user,or alternatively be statically set. The high pass filter 111 maycomprise a resonant high pass filter. The high pass filter 111 mayfurther comprise any circuits or combinations thereof structured to passthrough high frequencies above a first frequency, and attenuate orfilter out the lower frequencies.

The first processing module 105 is configured to process the filteredsignal 211 and transmit a partially processed signal 205 to the splitter121. Accordingly, the first processing module 105 may comprise a firstcompressor 112 in at least one embodiment. The first processing module105 may also comprise both a first compressor 112 and a clipping module113 in other embodiments.

In at least one embodiment, the first compressor 112 is configured tomodulate the filtered signal 211 and transmit a modulated signal 212 tothe clipping module 113. In other embodiments, the first compressor 112may transmit the modulated signal 212 as the partially processed signal205 to the splitter 112. The first compressor 112 may comprise anautomatic gain controller. The first compressor 112 may comprisestandard dynamic range compression controls such as threshold, ratio,attack and release. Threshold allows the first compressor 112 to reducethe level of the filtered signal 211 if its amplitude exceeds a certainthreshold. Ratio allows the first compressor 112 to reduce the gain asdetermined by a ratio. Attack and release determines how quickly thefirst compressor 112 acts. The attack phase is the period when the firstcompressor 112 is decreasing gain to reach the level that is determinedby the threshold. The release phase is the period that the firstcompressor 112 is increasing gain to the level determined by the ratio.The first compressor 112 may also feature soft and hard knees to controlthe bend in the response curve of the output or modulated signal 212,and other dynamic range compression controls appropriate for the dynamiccompression of an audio signal. The first compressor 112 may furthercomprise any device or combination of circuits that is structured andconfigured for dynamic range compression.

The clipping module 113, in at least one embodiment, is configured tolimit the gain of the modulated signal 212 and transmit a partiallyprocessed signal 213 to the splitter 121. Specifically, in at least oneembodiment, the clipping module 113 is configured to limit or removetransients from the modulated signal 212. A transient is a short burstof gain or amplitude in the audio signal. Transients may be generatedif, for instance, the first compressor used slower attack times in thecreation of the modulated signal 212. This would ensure minimum dynamicrange of the partially processed signal 213. A minimum dynamic range mayfurther facilitate the transmission of a signal over a communicationnetwork due to its smaller bandwidth or file size. The clipping module113 may comprise a clipper or clipping circuit. The clipping module 113may comprise series clippers, shunt clippers, or any combination ofcircuits capable and appropriate for clipping or limiting amplitude orgain of an audio signal. Additionally, the clipping module 113 may alsocomprise soft clipping, or devices or circuits capable of soft clippingin order to avoid sharp points in the transfer characteristic of asignal.

The splitter 121 is configured to split the partially processed signal213 into a first signal 221 and a second signal 221′. The first signal221 is transmitted to a low pass filter 122, while the second signal221′ is transmitted to a pass through module 123. In at least oneembodiment, the splitter 121 is configured such that the first signal221 and second signal 221′ are substantially the same signal as thepartially processed signal 213, i.e. correspondingly identical amplitudeand phase. In other embodiments, slight variations may result due tosignal degradation.

The low pass filter 122 is configured to pass through low frequencies ofthe first signal 221, while attenuating higher frequencies, based on asecond frequency. In other words, the frequencies below the secondfrequency are transmitted as a filtered first signal 222 to the mixer124. In at least one embodiment, and counterintuitive to expectedfunction, the second frequency is set below that of the first frequency.In voice applications the second frequency can be selected from a rangebetween 700 Hz and 1000 Hz. The second frequency may further be tunableby a user, or alternatively be statically set. The resultant firstfiltered signal 222 is used to rebuild usable dynamics from the lowfrequencies.

The pass through module 123 is configured to pass through the secondsignal 221′. In at least one embodiment, the pass through module 123will also adjust the gain of the second signal. For instance, in voiceapplications only a small amount of the signal is sufficient to achievedesired results, and the gain of the second signal 221′ will be adjusteddownward accordingly. One example of the pass through module 123 may beto restore sibilance in voice applications. Other examples may includethe restoration of other high frequency content if the low pass filter122 removes too much. In at least one embodiment, the level or gainadjustment of the pass through module 123 is a static adjustment. Thegain adjustment may also be zero. In other embodiments, dynamic oradjustable gain reduction may be desired and used. In at least oneembodiment, the pass through module may simply pass through a signalwithout any modification.

The second processing module 106 is configured to process the filteredfirst signal 222 and the gain-adjusted second signal 223 and transmit anoutput signal to the output device 102. Accordingly, the secondprocessing module 106 may comprise a mixer 124, a tone control module125, a second compressor 126, and any combinations thereof.

The mixer 124 is configured to combine the filtered first signal 222 andthe gain-adjusted second signal 223 and transmit a combined signal 224to the tone control module 125 in at least one embodiment. In someembodiments the combined signal 224 is transmitted as the output signal206 of the second processing module 106. In other embodiments thecombined signal 224 is further transmitted to a tone control module 125.The mixer 124 may comprise an electronic mixer structured to combine twoor more signals into a composite or combined signal. The mixer 124 maysimilarly comprise any circuit or combination of circuits structured orconfigured to combine two or more signals.

The tone control module 125 processes the combined signal 224 andtransmits a controlled signal 225 to the second compressor 126 in atleast one embodiment. In some embodiments the controlled signal 225 istransmitted as the output signal 206, in other embodiments thecontrolled signal 225 is further transmitted to the second compressor126. In at least one embodiment, the tone control module 125 comprises aparametric equalizer. Further, the parametric equalizer may compriseone, two, three, or four bands. In other embodiments, the tone controlmodule 125 may comprise a graphic equalizer. The tone control module 125may further be any device or combination of circuits appropriate for theequalization of an audio signal.

The second compressor 126, in at least one embodiment, is configured tomodulate the controlled signal 225 and transmit an output signal 226 tothe output device 102. The second compressor 126 may be similar instructure and/or configuration as the first compressor 112. Accordingly,the second compressor 126 may comprise any device or combination ofcircuits that is structured and configured for dynamic range compressionor static compression or level adjustment.

The output device 102 may be structured to further process the outputsignal 226. The output device 102 may also be structured and/orconfigured for playback of the output signal 226.

As schematically represented, FIG. 2 illustrates at least one preferredembodiment of a system 100 for narrow bandwidth digital signalprocessing in broadcasting applications. Accordingly, and in theseembodiments, the system 200 generally comprises the same or similarcomponents as system 100 as illustrated in FIG. 1, namely an inputdevice 101, a high pass filter 111, a first compressor 112, a clippingmodule 113, a splitter 121, a resonant low pass filter 122, a passthrough module 123, a mixer 124, a tone control module 125, a secondcompressor 126, and an output device 102. In the broadcasting embodimentof FIG. 2 as illustrated by system 200, these components may be splitbetween a pre-transmission module 110 and a post-transmission module120. Accordingly, the system 200 includes the addition of a transmitter131, a communications network 132, and a receiver 133 in between thepre-transmission module 110 and post-transmission module 120.

In effect, the embodiment of system 200 separates out various componentsof the present invention into at least two modules, such that thepre-transmission module or modules 110 and post-transmission module ormodules 120 may be placed in different locations or embedded intodifferent devices in communication with one another. The network 132 maycomprise a wired or wireless network, such as cellular, satellite,terrestrial, and any variety of LAN, WAN, local wireless or near fieldcommunications including but not limited to Wifi, Bluetooth, infraredtechnologies, as well as any other appropriate technologies for thetransmission of a digital audio signal. Accordingly, the transmitter 131and corresponding receiver 133 are configured to send and receive thepartially processed signal 213 from the pre-transmission module 110 tothe post-transmission module 120. In the embodiment of FIG. 2, thepartially processed signal 213 is transmitted from the clipping module113 to the transmitter 131, sent over the network 132 to the receiver133, and then is transmitted to the splitter 121.

As schematically represented, FIG. 3 illustrates at least one preferredembodiment of a system 300 for narrow bandwidth digital signalprocessing. Generally speaking, the system 300 is a simpler variation ofthe embodiment of system 100, and has fewer components than the system100 recited above. As such, the system 300 comprises an input device101, a high pass filter 111, a first compressor 112, a splitter 121, alow pass filter 122, a pass through module 123, a mixer 124, and anoutput device 102. However, it should be understood that the system 300embodiment may comprise the addition of the clipping module 113, tonecontrol module 125, second compressor 126, and any combinations thereof.

Structurally and in at least one embodiment, the components recited insystem 300 are similar to or the same as the components in system 100above. Modulated signal 212 from the first compressor 112 however, istransmitted directly to the splitter 121. Further, the combined signal224 from mixer 124 is sent directly to the output device 102.

As schematically represented, FIG. 4 illustrates at least one preferredembodiment of a broadcast variation of the system of FIG. 3. In thisembodiment, system 400 comprises a pre-transmission module 110′ and apost-transmission module 120′. The pre-transmission module 110′comprises a high pass filter 111 and a first compressor 112. Thepost-transmission module 120′ comprises a splitter 121, a low passfilter 122, a pass through module 123, and a mixer 124. Structurally,the components are as those recited in system 300 and system 200 abovein at least one embodiment.

As diagrammatically represented, FIG. 5 illustrates another embodimentdirected to a method for narrow bandwidth processing, which may, invarious embodiments, incorporate the components from the systemsreferenced above. In this embodiment, an input audio signal is firstfiltered, as in 501, with a high pass filter. In at least oneembodiment, the high pass filter comprises a resonant filter that passesfrequencies above a first frequency and attenuates frequencies below thefirst frequency. Further, the first frequency may be selected from arange between 2.5 kHz and 3.2 kHz.

The filtered signal comprising frequencies above the first frequency arethen processed by the first processing module, as in 1001, to create apartially processed signal. This step may further comprise modulatingthe filtered signal with a first compressor to create a modulatedsignal. The first compressor may comprise an automatic gain controller,or other device or combination of circuits capable of dynamic rangecompression. In some embodiments, the first compressor may comprise anydevice or combination of circuits capable of static level adjustments.The modulated signal may be transmitted as the partially processedsignal, or may further be transmitted to a clipping module to limit thegain of the modulated signal in order to create the partially processedsignal. The clipping module may comprise a clipper or clipping circuitconfigured to limit the amplitude or gain of an audio signal. Theclipping module may further comprise soft clipping in order to avoidsharp points in the transfer characteristics of the signal.

The partially processed signal is then split using a splitter, as in504, into a first signal and a second signal. In at least oneembodiment, the splitter is configured such that the first and secondsignals are substantially the same signal as the partially processedsignal. In other words, the partially processed signal diverges into afirst signal and a second signal at the splitter.

The first signal is filtered, as in 505, with a low pass filter. In atleast one embodiment, the low pass filter comprises a resonant filterthat passes frequencies below a second frequency and attenuatesfrequencies above the second frequency. Further, and counterintuitive toexpected function, the second frequency is set to be lower than thefirst frequency. In at least one embodiment, the second frequency mayfurther be selected from the range between 700 Hz and 1000 Hz.

The second signal is passed through the pass through module, as in 506.The gain of the second signal is adjusted in at least one embodiment.The gain adjustment may be zero, may be adjusted downwards or upwards.In some embodiments the gain adjustment may be static, in othersdynamic. In yet other embodiments, the pass through module may notcomprise a gain adjustment component. Accordingly, the pass throughmodule may comprise a device or circuit(s) for the dynamic or staticgain adjustment of the signal.

The filtered first signal and gain-adjusted second signal are thenprocessed with a second processing module, as in 1002. This step mayfurther comprise combining the filtered first signal and thegain-adjusted second signal with a mixer in order to create a combinedsignal. The mixer is structured and/or configured to combine at leasttwo signals and output a composite signal. The combined signal may betransmitted as the output signal, or may be further processed with atone control module to create a controlled signal. The tone controlmodule may comprise any device or combination of circuits appropriatefor the equalization of an audio signal. In at least one embodiment, thetone control module comprises a parametric equalizer. The parametricequalizer may comprise one, two, three, or four bands. In otherembodiments, the tone control module may comprise graphic equalizers orother equalizers. The controlled signal may be output as the outputsignal, or may be further modulated with a second compressor to createthe output signal. The second compressor may comprise any device orcombination of circuits capable of dynamic range compression or staticcompression.

Finally, the output signal is transmitted to an output device, as in510. The output device may be configured for additional processing, ormay be configured for playback of the output signal.

As diagrammatically represented, FIG. 6 illustrates another embodimentdirected to a broadcasting variation of a method for narrow bandwidthprocessing. Structural and or configurable components of this embodimentmay be the same or similar as those recited above.

In the embodiment of FIG. 6, an input audio signal is filtered, as in501, with a high pass filter. In at least one embodiment, the high passfilter comprises a resonant filter that passes frequencies above a firstfrequency and attenuates frequencies below the first frequency. Further,the first frequency may be selected from a range between 2.5 kHz and 3.2kHz.

The filtered signal is then processed by the first processing module, asin 1001. This step may comprise modulation by a first compressor,limiting the gain with a clipping module, or combinations thereof, inorder to create a partially processed signal.

The partially processed signal is then transmitted with a transmitter,as in 601. The transmitter may comprise any device or combination ofcircuits capable of transmitting a signal, wired or wireless, over acommunication network or platform. The partially processed signal isreceived with a receiver, as in 602. The receiver may comprise anydevice or combination of circuits capable of receiving the partiallyprocessed signal being transmitted by the transmitter.

The partially processed signal received by the receiver is then splitinto a first signal and second signal, as in 504, with a splitter.

The first signal is filtered with a low pass filter, as in 505. In atleast one embodiment, the low pass filter comprises a resonant filterthat passes frequencies below a second frequency and attenuatesfrequencies above the second frequency. Further, and counterintuitive toexpected function, the second frequency is set to be lower than thefirst frequency. In at least one embodiment, the second frequency mayfurther be selected from the range between 700 Hz and 1000 Hz.

The second signal is passed through a pass through module, as in 506. Inat least one embodiment, the gain of the second signal is adjusted withthe pass through module.

The filtered first signal and gain-adjusted second signal are thenprocessed with a second processing module, as in 1002. This step maycomprise combination of the two signals with a mixer, further processingwith a tone control module, modulating with a second compressor, andcombinations thereof, in order to create the output signal. Finally, thesecond processing module transmits an output signal as in 510 to theoutput device.

As diagrammatically represented, FIG. 7 illustrates another embodimentdirected to a method for narrow bandwidth processing. Structural and orconfigurable components of this embodiment may be the same or similar asthose recited above. In the embodiment of FIG. 7, an input audio signalis filtered, as in 701, with a high pass filter. In at least oneembodiment, the high pass filter comprises a resonant filter that passesfrequencies above a first frequency and attenuates frequencies below thefirst frequency. Further, the first frequency may be selected from arange between 2.5 kHz and 3.2 kHz.

The filtered signal is then modulated by a first compressor, as in 702,in order to create a partially processed signal. The partially processedsignal is then split into a first signal and second signal, as in 703,with a splitter.

The first signal is filtered with a low pass filter, as in 704. In atleast one embodiment, the low pass filter comprises a resonant filterthat passes frequencies below a second frequency and attenuatesfrequencies above the second frequency. Further, and counterintuitive toexpected function, the second frequency is set to be lower than thefirst frequency. In at least one embodiment, the second frequency mayfurther be selected from the range between 700 Hz and 1000 Hz.

The second signal is passed through a pass through module, as in 705. Inat least one embodiment, the gain of the second signal is adjusted withthe pass through module. The filtered first signal and gain-adjustedsecond signal are combined in a mixer, as in 706. Finally, the combinedsignal is transmitted as an output signal, as in 707.

As diagrammatically represented, FIG. 8 illustrates another embodimentdirected to a broadcasting variation of a method for narrow bandwidthprocessing. Structural and or configurable components of this embodimentmay be the same or similar as those recited above.

In the embodiment of FIG. 8, an input audio signal is filtered, as in701, with a high pass filter. In at least one embodiment, the high passfilter comprises a resonant filter that passes frequencies above a firstfrequency and attenuates frequencies below the first frequency. Further,the first frequency may be selected from a range between 2.5 kHz and 3.2kHz. The filtered signal is then modulated by a first compressor, as in702, in order to create a partially processed signal.

The partially processed signal is then transmitted with a transmitter,as in 801. The transmitter may comprise any device or combination ofcircuits capable of transmitting a signal, wired or wireless, over acommunication network or platform. The partially processed signal isreceived with a receiver, as in 802. The receiver may comprise anydevice or combination of circuits capable of receiving the partiallyprocessed signal being transmitted by the transmitter.

The partially processed signal received by the receiver is then splitinto a first signal and second signal, as in 703, with a splitter.

The first signal is filtered with a low pass filter, as in 704. In atleast one embodiment, the low pass filter comprises a resonant filterthat passes frequencies below a second frequency and attenuatesfrequencies above the second frequency. Further, and counterintuitive toexpected function, the second frequency is set to be lower than thefirst frequency. In at least one embodiment, the second frequency mayfurther be selected from the range between 700 Hz and 1000 Hz.

The second signal is passed through a pass through module, as in 705. Inat least one embodiment, the gain of the second signal is adjusted withthe pass through module. The filtered first signal and gain-adjustedsecond signal are combined in a mixer, as in 706. Finally, the combinedsignal is transmitted as an output signal, as in 707.

The above methods may be completed in sequential order in at least oneembodiment, though they may be completed in any other order. In at leastone embodiment, the above methods may be exclusively performed, but inother embodiments, one or more steps of the methods as described may beskipped.

Since many modifications, variations and changes in detail can be madeto the described preferred embodiment of the invention, it is intendedthat all matters in the foregoing description and shown in theaccompanying drawings be interpreted as illustrative and not in alimiting sense. Thus, the scope of the invention should be determined bythe appended claims and their legal equivalents.

Now that the invention has been described,

What is claimed is:
 1. A system for narrow bandwidth digital signalprocessing comprising: a splitter configured to split the an leastpartially processed audio signal into a first signal and a secondsignal; a low pass filter configured to filter said first signal; a passthrough module configured to adjust a gain of said second signal; and asecond processing module configured to process said filtered firstsignal and said gain-adjusted second signal to create an output signal,said second processing module including at least a mixer configured tocombine said filtered first signal and said gain-adjusted second signal,creating a combined signal.
 2. The system as recited in claim 1 whereinsaid second processing module comprises a tone control module configuredto process said combined signal, creating a controlled signal; and asecond compressor configured to modulate said controlled signal tocreate an output signal.
 3. A system as recited in claim 1 furthercomprising a first processing module including a first compressorconfigured to modulate an input audio signal, creating said partiallyprocessed signal.
 4. A system as recited in claim 3 wherein said firstprocessing module further comprises a clipping module configured toadjust the gain of said modulated signal, creating said partiallyprocessed signal.
 5. A system as recited in claim 3 further comprising ahigh pass filter configured to filter said input audio signal to createa filtered signal based on a first frequency.
 6. A system as recited inclaim 5 wherein said first frequency is tunable.
 7. A system as recitedin claim 5 wherein said first frequency is selected from the rangebetween 2.5 kHz and 3.2 kHz.
 8. A system as recited in claim 1 whereinsaid low pass filter is configured to filter said first signal based ona second frequency.
 9. A system as recited in claim 8 wherein saidsecond frequency is tunable.
 10. A system as recited in claim 8 whereinsaid second frequency is lower than said first frequency.
 11. A systemas recited in claim 8 wherein said second frequency is selected from therange between 700 Hz and 1000 Hz.
 12. A system as recited in claim 2wherein said tone control module comprises a parametric equalizer.
 13. Amethod for narrow bandwidth digital signal processing of an input audiosignal comprising: processing the input audio signal with apre-transmission module, creating a partially processed signal;transmitting the partially processed signal to a receiver; splitting thereceived signal into a first signal and a second signal; filtering thefirst signal to create a filtered first signal; adjusting the gain ofthe second signal to create a gain-adjusted second signal; combining thefiltered first signal and gain-adjusted second signal to create acombined signal; and modulating the controlled signal to create anoutput signal.
 14. The method as recited in claim 13 further comprisingfiltering the input audio signal to create a filtered signal; processingthe filtered signal to create the partially processed signal.
 15. Themethod as recited in claim 14 further comprising modulating the filteredsignal with a first compressor to create a modulated signal.
 16. Themethod as recited in claim 15 further comprising adjusting the gain ofthe modulated signal to create the partially processed signal.
 17. Asystem as recited in claim 13 further comprising filtering the inputaudio signal using a high pass filter based on a first frequency,creating a first signal.
 18. The method as recited in claim 17 whereinthe first frequency is selected from the range between 2.5 kHz and 3.2kHz.
 19. A system as recited in claim 17 further comprising filteringthe first signal with a low pass filter configured to filter the firstsignal based on a second frequency.
 20. A system as recited in claim 19wherein the second frequency is lower than the first frequency.